As used herein, the term "arrhythmia" refers to any abnormal heart rhythm that may be dangerous to the patient and specifically includes fibrillation, tachycardias, supraventricular tachycardias (SVT), ventricular tachycardias (VT), ventricular fibrillation and flutter (VF) and bradycardia. As further used herein, the term "therapy" refers to any means used by the ICD device to restore normal heart rhythm, such as defibrillation, cardioversion, antitachycardia pacing and drug infusion. The disclosed invention has application to ICD devices which treat tachyarrhythmias (abnormally high heart rate) and/or bradyarrhythmias (low heart rate).
Modern conventional bradycardia pacemakers all have sensing mechanisms to enable the device to inhibit pacing when the heart is beating normally. Implantable tachyarrhythmia devices must also sense the heart's electrical activity, known as the intracardiac electrogram (ICEG), to determine whether the patient needs treatment. Intracardiac electrograms exhibit highly variable amplitudes during transitions from normal sinus rhythm to rhythms such as ventricular tachycardia and ventricular fibrillation. Because of this, it is advantageous that such sensing systems have the ability to automatically adjust to the changing amplitude of the signal.
U.S. Pat. No. 4,184,493 to Langer et al., which issued on Jan. 22, 1980, and is entitled "Circuit for Monitoring a Heart and for Effecting Cardioversion of a Needy Heart" describes such a sensing circuit that automatically adjusts to the amplitude of the heart's electrical signal using a conventional feedback automatic gain system. One problem with this form of automatic sensing is that its response time is slow, due to the damping required to maintain stability of the feedback loop.
Another form of automatic sensing system is described in U.S. Pat. No. 4,903,699 to Baker et al., which issued on Feb. 27, 1990, and is entitled "Implantable Cardiac Stimulator With Automatic Gain Control". The Baker et al. patent uses a system of comparators and adjustable thresholds to optimally detect the ICEG signal. As in the case of the above Langer et al. patent, the response time of this system is slow, thereby resulting in a reduction of speed and efficiency of the sensing system.
Both of the foregoing patents describe systems which filter the ICEG signal to remove low frequency noise and artifacts. High pass filtering is also required to prevent sensing of T-Waves during normal sinus rhythm (NSR). A problem with these techniques is that the high pass filtering attenuates the signal during VF, since the VF signal is often likely to have a spectral content similar to that of T-Waves.
It is, therefore, a primary object of this invention to provide an improved cardiac signal detection system having a fast response to a patient's electrical cardiac activity in rhythms such as NSR, SVT, VT and VF.
It is a further object of this invention to provide reliable sensing of cardiac activity when the amplitude of the ICEG signal is varying rapidly.
It is another object of this invention to sense only the QRS complex of the ICEG during normal sinus rhythm of the heart, without sensing T-Waves, in order to prevent VF attenuation.
It is a still further object of the invention to provide a method and apparatus for optimally sensing the intracardiac electrogram with an implantable antiarrhythmia device that provides optimum sensing with different levels of input signals and different signal morphologies while being immune to false sensing of T-Waves and other artifacts.
Further objects and advantages of this invention will become apparent as the following description proceeds.